io uso questa per pop up centrati:
codice:
<script type="text/javascript">
<!--
var win= null;
function NewWindow(mypage,myname,w,h,scroll,toolbar){
var winl = (screen.width-w)/2;
var wint = (screen.height-h)/2;
var settings ='height='+h+',';
settings +='width='+w+',';
settings +='top='+wint+',';
settings +='left='+winl+',';
settings +='scrollbars='+scroll+',';
settings +='toolbar='+toolbar+',';
settings +='resizable=yes';
win=window.open(mypage,myname,settings);
if(parseInt(navigator.appVersion) >= 4){win.window.focus();}
}
//-->
</script>
nel link